test: lint EXX error codes #8155

Problem:
- Choosing a new EXX error code is tedious.
- It's possible to accidentally use an EXX error code for different
  purposes.

Solution:
Add a lint check which requires EXX error codes to have a :help tag.
This also avoids duplicates because `make doc` does `:helptags ++t doc`
which fails if duplicates are found.

(cherry picked from commit bc6d946cca)

scripts/linterrcodes.lua

@@ -0,0 +1,241 @@
+-- Checks mismatches between "EXX" error codes (E123, E1234) defined in C sources and those
+-- documented in `runtime/doc/*.txt`.
+--
+-- Usage: nvim -l scripts/linterrcodes.lua
+
+--- Error codes allowed to appear in more than one place. Value is the exact expected
+--- occurrence count. A mismatch (actual > or < expected) is reported, to avoid
+--- accidental duplicates from slipping in.
+--- @type table<string, integer>
+local dup_allowed = {
+  E109 = 2,
+  E1098 = 2,
+  E110 = 2,
+  E112 = 2,
+  E114 = 2,
+  E115 = 2,
+  E1159 = 2,
+  E116 = 2,
+  E121 = 2,
+  E1502 = 4,
+  E151 = 2,
+  E155 = 3,
+  E158 = 2,
+  E170 = 2,
+  E173 = 2,
+  E180 = 2,
+  E212 = 2,
+  E216 = 2,
+  E298 = 3,
+  E303 = 3,
+  E312 = 2,
+  E317 = 4,
+  E319 = 2,
+  E423 = 3,
+  E474 = 52,
+  E475 = 6,
+  E482 = 3,
+  E484 = 2,
+  E488 = 2,
+  E5000 = 2,
+  E5001 = 2,
+  E5002 = 2,
+  E5009 = 3,
+  E502 = 2,
+  E503 = 3,
+  E504 = 2,
+  E505 = 2,
+  E509 = 2,
+  E5101 = 2,
+  E5102 = 2,
+  E5108 = 4,
+  E5111 = 2,
+  E513 = 2,
+  E521 = 2,
+  E546 = 2,
+  E588 = 2,
+  E678 = 2,
+  E685 = 5,
+  E697 = 2,
+  E703 = 2,
+  E716 = 2,
+  E723 = 2,
+  E724 = 3,
+  E728 = 2,
+  E741 = 2,
+  E742 = 2,
+  E745 = 2,
+  E798 = 2,
+  E805 = 2,
+  E856 = 2,
+  E867 = 2,
+  E900 = 3,
+  E903 = 2,
+  E905 = 2,
+  E906 = 2,
+  E948 = 2,
+  E970 = 2,
+  E974 = 2,
+  E996 = 5,
+}
+
+--- Runs a command, returns stdout lines. Errors on non-zero exit.
+--- @param cmd string[]
+--- @return string[]
+local function run(cmd)
+  local result = vim.system(cmd, { text = true }):wait()
+  if result.code ~= 0 then
+    error('command failed: ' .. table.concat(cmd, ' ') .. '\n' .. (result.stderr or ''))
+  end
+  return vim.split(result.stdout, '\n', { trimempty = true })
+end
+
+--- Extracts error codes from a line of C source, excluding hex literals (0xE000),
+--- identifiers (FOO_E123), and inline comments (`//`).
+--- @param line string
+--- @return string[]
+local function extract_codes(line)
+  local codes = {} --- @type string[]
+  local cmt = line:find('//')
+  for pos, code in line:gmatch('()(E%d%d%d%d?)') do
+    --- @cast pos integer
+    local in_comment = cmt and pos > cmt
+    -- Preceded by a word char means the `E` is part of something else.
+    local prev = pos > 1 and line:sub(pos - 1, pos - 1) or ''
+    local in_word = prev:match('[%w_]') ~= nil
+    if not in_comment and not in_word then
+      codes[#codes + 1] = code
+    end
+  end
+  return codes
+end
+
+--- @return table<string, true> Set of error codes documented in help docs.
+local function collect_help_codes()
+  local lines = run({
+    'git',
+    'grep',
+    '-hE',
+    [[\*E[0-9]{3,4}\*]],
+    '--',
+    'runtime/doc/*.txt',
+  })
+  local codes = {} --- @type table<string, true>
+  for _, line in ipairs(lines) do
+    for code in line:gmatch('E%d%d%d%d?') do
+      codes[code] = true
+    end
+  end
+  return codes
+end
+
+--- @return table<string, string[]> Map of error code to its occurrences in C sources.
+local function collect_c_codes()
+  local lines = run({
+    'git',
+    'grep',
+    '-nE',
+    'E[0-9]{3,4}',
+    '--',
+    'src/nvim/*.c',
+    'src/nvim/*.h',
+  })
+  local codes = {} --- @type table<string, string[]>
+  for _, line in ipairs(lines) do
+    for _, code in ipairs(extract_codes(line)) do
+      codes[code] = codes[code] or {}
+      table.insert(codes[code], line)
+    end
+  end
+  return codes
+end
+
+--- @param a string
+--- @param b string
+--- @return boolean
+local function errcode_lt(a, b)
+  return tonumber(a:sub(2)) < tonumber(b:sub(2))
+end
+
+--- @param c_codes table<string, string[]>
+--- @param help_codes table<string, true>
+--- @return integer missing Number of codes missing from help docs.
+--- @return integer dups Number of codes with unexpected duplicate usage.
+local function report(c_codes, help_codes)
+  local missing = {} --- @type string[]
+  for code in pairs(c_codes) do
+    if not help_codes[code] then
+      missing[#missing + 1] = code
+    end
+  end
+  table.sort(missing, errcode_lt)
+
+  local dup_codes = {} --- @type string[]
+  for code, occurrences in pairs(c_codes) do
+    local allowed = dup_allowed[code]
+    if allowed then
+      -- Whitelisted: only flag if the actual count doesn't match the expected count.
+      if #occurrences ~= allowed then
+        dup_codes[#dup_codes + 1] = code
+      end
+    elseif #occurrences > 1 then
+      dup_codes[#dup_codes + 1] = code
+    end
+  end
+  table.sort(dup_codes, errcode_lt)
+
+  if #missing > 0 then
+    print('Error codes missing from help docs:')
+    for _, code in ipairs(missing) do
+      print('  ' .. code)
+    end
+    print('')
+  end
+
+  if #dup_codes > 0 then
+    print('Error codes used in more than one place:')
+    for _, code in ipairs(dup_codes) do
+      print(string.format('  %s (%d occurrences):', code, #c_codes[code]))
+      for _, loc in ipairs(c_codes[code]) do
+        print('    ' .. loc)
+      end
+    end
+    print('')
+  end
+
+  local max_code = 0
+  for code in pairs(c_codes) do
+    local n = tonumber(code:sub(2)) or 0
+    if n > max_code then
+      max_code = n
+    end
+  end
+
+  local n_errcodes = 0
+  for _ in pairs(c_codes) do
+    n_errcodes = n_errcodes + 1
+  end
+
+  print(
+    string.format(
+      'errcodes=%d dup-codes=%d missing-help=%d highest=E%d',
+      n_errcodes,
+      #dup_codes,
+      #missing,
+      max_code
+    )
+  )
+
+  return #missing, #dup_codes
+end
+
+local function main()
+  local help_codes = collect_help_codes()
+  local c_codes = collect_c_codes()
+  local missing, dups = report(c_codes, help_codes)
+  if missing > 0 or dups > 0 then
+    os.exit(1)
+  end
+end
+
+main()